Here I will explain how to solve problem of “The type or namespace name 'CrystalDecisions' could not be found (are
you missing a using directive or an assembly reference?” in visual studio
2010 while using crystal reports in windows application in c#, vb.net.

Actually
this problem occurs because of pointing our application to .NET Framework 4 Client Profile like as shown below to check this
Right Click on your Applicationà Go to Properties à Select Application Tab

To
fix this problem we need to change Target
Framework version from .NET
Framework 4 Client Profile to .NET
Framework4

Once we
change our Target Framework version to .NET
Framework4 build the application and check it will fix the error.